Question: Let $a,$ $b,$ $c,$ $d,$ $e,$ $f,$ $g,$ and $h$ be real numbers such that $abcd = 4$ and $efgh = 9.$  Find the minimum value of
\[(ae)^2 + (bf)^2 + (cg)^2 + (dh)^2.\]
Explanation: By AM-GM,
\begin{align*}
(ae)^2 + (bf)^2 + (cg)^2 + (dh)^2 &\ge 4 \sqrt[4]{(ae)^2 (bf)^2 (cg)^2 (dh)^2} \\
&= 4 \sqrt[4]{(abcdefgh)^2} \\
&= 24.
\end{align*}Equality occurs when $(ae)^2 = (bf)^2 = (cg)^2 = (dh)^2,$ $abcd = 4,$ and $efgh = 9.$  For example, we can take $a = b = c = d = \sqrt{2}$ and $e = f = g = h = \sqrt{3}.$  Hence, the minimum value is $\boxed{24}.$